1. personal health services—those provided for individuals by doctors, nurses, and health technicians of many kinds to treat illness, prevent disease or disability, or facilitate such normal processes as human reproduction; 2. environmental health activities—those provided on a mass basis by engineers, sanitarians, and others to minimize disease and discomfort by controlling the quality of specific environmental elements such as water, food, air, and housing; 3. public information and education—those provided by individual and mass means to increase public knowledge of health and of conditions affecting it; 4. vital registration and health surveillance—those provided through individual records, sample surveys, and other means to document key events during life, to identify hazardous diseases and conditions, and to measure the level of health and health services in the population. It is not possible to cover adequately the effects of population growth on all of these major types of activities, nor can one assume that effects will be identical for all of them. The effects of numbers of people alone depend upon the type of service. If it is to individuals, an increase in numbers requires a more or less direct increase in service. If the service is a mass approach to sanitation or information, an increase in numbers may call for little extra service effort; in fact a minimum population size may even be necessary before the service can be started. Classifying services into those controlled by the public sector and those controlled by the private sector is important within most countries and essential for comparative analyses of different countries. However, obtaining comparable data for both sectors is diffiuclt within most countries, and much more so between countries. These differences and difficulties are as great among the MDC's as the LDC's. Personal health services are about equally important for mortality and natality control; they present most of the range of considerations needed to understand the effect of population growth on other health services; and they have data available to consider. Therefore, these services have been selected for attention in this paper. LEVELS OF DEVELOPMENT A country is defined as more or less developed at a given time in history on the basis of whether it falls above or below an arbitrary value for one or more of an arbitrary list of interrelated characteristics. Characteristics commonly used (where relevant, on a per capita or family basis) include gross national product (GNP), power consumption, and others shown in Table 1. For this paper, per capita income of less than $600 (U.S.) in 1966 qualified a country as less developed. To provide some comparison with MDC's, data on seven MDC's are included in Table 2.r good health or the services required far outweigh immediate relevance.